lavc: add a dummy field to AVStream to preserve ABI compatibility for avconv

avconv abuses the API by accessing AVStream.parser (which is private).
Removing AVStream.reference_dts in
2ba68dd044ca8fc591139c05563840f546a9c0c0 breaks ABI compatibility for an
old avconv using a newer lavf. Fix this by adding a dummy field until
the next bump.

libavformat/avformat.h

@@ -729,6 +729,11 @@ typedef struct AVStream {
 
     int pts_wrap_bits; /**< number of bits in pts (used for wrapping control) */
 
+#if FF_API_REFERENCE_DTS
+    /* a hack to keep ABI compatibility for avconv, which accesses parser even
+     * though it should not */
+    int64_t do_not_use;
+#endif
     // Timestamp generation support:
     int64_t first_dts;
     int64_t cur_dts;

libavformat/version.h

@@ -48,5 +48,8 @@
  * dropped at a future version bump. The defines themselves are not part of
  * the public API and may change, break or disappear at any time.
  */
+#ifndef FF_API_REFERENCE_DTS
+#define FF_API_REFERENCE_DTS            (LIBAVFORMAT_VERSION_MAJOR < 56)
+#endif
 
 #endif /* AVFORMAT_VERSION_H */